1. Field of the Invention
The present invention relates to a turbo-charged engine with EGR (Exhaust Gas Recirculation).
2. Description of the Related Art
A type shown in FIG. 3 is known as a turbo-charged engine with EGR (see Japanese Patent Applications published with Nos. 9-137754 and 2000-249004, for example).
As illustrated, a turbine c is arranged in an exhaust passage b of a diesel engine a, and a compressor e driven by the turbine c is arranged in an intake passage d. An EGR passage f is connected to the exhaust passage b upstream of the turbine c and the intake passage d downstream of the compressor e, so that some of the exhaust gas in the exhaust passage b flows back to the intake passage d through the EGR passage f.
An EGR valve g, an EGR cooler h and a check valve i are provided in the EGR passage f. The EGR valve g adjusts a passage area of the EGR passage f. The EGR cooler cools the exhaust gas (EGR gas) flowing in the EGR passage f. The check valve i allows a flow only in a direction from the exhaust passage b side to the intake passage d side, and prevents the flow in the opposite direction thereof. A charge cooler j for cooling intake air is provided in the intake passage d downstream of the compressor e.
According to such turbo-charged engine with EGR, EGR is performed by opening the EGR valve g to allow some of exhaust gas in the exhaust passage b to flow toward the intake passage d through the EGR passage f. Reverse flow in the EGR passage f (or intake air flow from the intake passage d to the exhaust passage b) is prevented by the check valve i.
Conventionally, it is common that EGR is performed only in a low load region and is not performed in a high load region with the EGR valve g closed. The reason for this is described below. In the low load region, even if the EGR gas is inhaled by the engine, smoke and fuel consumption are not worsened and power output does not become lower, and the NOx reduction effect as an inherent effect of EGR can be acquired, because an amount of air inhaled by the engine is sufficient to an amount of injection fuel. However, in the higher load region, if the EGR gas is inhaled, the fuel consumption is worsened and the power output becomes lower and the smoke tends to be generated, because margin of the amount of intake air relative to the amount of the injection fuel becomes less in the higher load region and an amount of air (oxygen) becomes relatively insufficient.
On the other hand, there is a requirement in recent years that the fuel consumption and the exhaust gas should be improved, therefore, high pressure supercharging is made forward especially in the diesel engine a. By raising supercharging pressure, an amount of intake air per cylinder can increase, power output can increase, and fuel consumption (fuel consumption per unit horsepower) can improve. However, recklessly raising the supercharging pressure makes operation of EGR difficult, caused by relation between exhaust gas pressure and intake air pressure. And also, performing EGR means not supplying energy of exhaust gas to the turbine c of the turbocharger k, which leads into difficulty of matching the turbocharger k with the engine a.
Thus, in recent years that the engine is highly supercharged, an optimal solution about matching between a setting of the turbocharger k and an EGR control is not clarified.
It is an object of the present invention to provide a turbo-charged engine with EGR which attains optimization about matching between a setting of the turbocharger and an EGR control with regard to highly supercharged engine.